This invention pertains generally to the field of thermal control films. More particularly, the present invention pertains to visibly transparent heat reflective thermal control films which comprise an optical coating and which are suitable for use in glazing applications. More specifically, the present invention pertains to coated polymer sheets which comprise a thin flexible polymeric sheet (which serves as the substrate), and at least one multilayer coating coated thereon, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material. The coated polymer sheets are characterized by a high transmission of visible radiation (i.e., visibly transparent) and a high reflectance at one or more near infrared radiation center wavelengths (i.e., heat reflective), and rely primarily on the interference effects of the dielectric layers to achieve these results.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The term xe2x80x9cglazingxe2x80x9d is used herein in its conventional sense and relates to the use of transparent materials (e.g., glass) to fill apertures as in, for example, windows, viewports, and the like. Vehicular glazing generally refers to the use of transparent materials such as windows, windshields, windscreens, canopies, panes, and the like, in vehicles such as automobiles, trains, boats, aircraft, and spacecraft. Architectural glazing generally refers to the use of transparent materials as windows, viewports, skylights, panes, and the like in buildings, such as domestic buildings and commercial buildings.
Conventional window glass, which has been used since medieval times as the glazing material of choice, is highly transparent to visible radiation. Visible light is transmitted through the glass and is absorbed or reflected by materials on the opposite side (e.g., floor, walls, flirniture, plants, and other objects in the interior of a building). These absorbing materials re-emit some of the absorbed radiation according to their temperature. Near ambient temperatures (i.e., xcx9c5 to 40xc2x0 C.), these objects have a blackbody peak emission in the infrared region at approximately 7-10 microns (see below). Although conventional glass is largely transparent between xcx9c0.2 microns and xcx9c3.5 microns, it is substantially opaque at infrared wavelengths greater than xcx9c4 microns, and the re-radiated heat (e.g., at 7-10 microns and longer wavelengths) is either absorbed by the glass or reflected back (e.g., into the interior of the building). This is the fundamental basis for glass-covered greenhouses.
While windows often enhance the aesthetics and futnctionality of buildings and vehicles, they can also cause undesirable gain or loss of heat. In warm climates, exterior heat may enter through windows, thereby increasing air conditioning loads. In cold climates, interior heat is lost through windows, thereby increasing heating demands. Increases in the size of windows used in automobiles coupled with reductions in the size of vehicular air conditioners have increased the need for vehicle windows which reduce heat load. See, for example, Chiou, 1986; Nyman, 1990; Huber, 1988; Hymore etal., 1991; Lynam, 1990.
Heat loss through a window may arise from a convective/conductive/emissive process, for example, where interior hot air raises the temperature of the glass, by convection, the thermal energy is distributed throughout the glass, by conduction, and some of the thermal energy is emitted or radiated, by emission, to the exterior. Heat loss by emission can be ameliorated by reducing the emissivity of the window glass, for example, by introducing a low emittance or xe2x80x9clow Exe2x80x9d (for infrared) coating (which is typically a thin metal film). Emissivity or emittance refers to the propensity of a surface to emit or radiate radiation of a specified wavelength, and is quantified as the ratio of radiant flux per unit area emitted by body to that of a blackbody radiator at the same temperature and under the same conditions. Thus, a perfect blackbody has an emissivity of 1.0. Ordinary window glass has an infrared emissivity of about 0.84. Window glass with a xe2x80x9clow Exe2x80x9d coating has a much lower infrared emissivity, often as low as 0.15, and heat loss through such a window is greatly reduced.
Optical coatings have found widespread application in the field of glazing, particularly as a means to control heat loss and/or heat gain. In many applications, optical coatings are used to xe2x80x9cblockxe2x80x9d the transmission of electromagnetic radiation (e.g., infrared radiation, visible radiation, ultraviolet radiation) to some degree. In some applications, it is desirable to block some or all of the electromagnetic radiation of a particular wavelength band while transmitting some or all of the electromagnetic radiation of another particular wavelength band.
Thus, in one conunon application, an optical coating is employed to substantially block infrared electromagnetic radiation while substantially transmitting visible electromagnetic radiation. Such optical coatings are often referred to as xe2x80x9cheat mirrors,xe2x80x9d xe2x80x9chot mirrors,xe2x80x9d or xe2x80x9cthermal control films.xe2x80x9d For glazing applications, it is usually desirable that these optical coatings also be substantially visibly transparent.
An important application for optical coatings is as thermal control films for solar radiation. The sun, which is the source of solar radiation, is a modest yellow star with a diameter of about 1.4 million kilometers at an average distance from the earth of about 150 billion kilometers. The sun has interior temperatures on the order of 8 to 40 million K and a surface temperature of about 5800 K. The rate of energy emission from the sun is about 3.8xc3x971023 kW, of which 1.7xc3x971014 kW is intercepted by the earth. Of this amount, 30% is reflected, 47% is converted into low temperature heat and re-radiated into space, and about 23% powers the evaporation and precipitation cycle of the earth""s biosphere. The extraterrestrial solar irradiance at normal incidence is about 1373 W/m2. At an air mass of one (see below), the irradiance is about 925 W/m2, the bulk of which falls in the wavelength band from about 200 nm (in the ultraviolet) to about 2000 nm (in the near infrared).
To accurately predict solar intensity in the visible region, the sun may be characterized as a blackbody with a temperature e of approximately 5800 K. To accurately predict solar intensity in the infrared region, the sun may be characterized as a blackbody with a temperature of approximately 5900 K. Blackbody radiation may be modeled using a Planck distribution, according to which the energy density in the range xcex to xcex+dxcex, denoted dU(xcex), is given by:
dU(xcex)=[(8xcex7hc/xcex5)(exe2x88x92hc/xcexkT)/(1-exe2x88x92hc/xcexkT)]dxcex
wherein T is the blackbody temperature, xcex is the wavelength, h is the Planck constant, c is the speed of light, and k is the Boltzmann constant. The wavelength of peak emission, xcexmax, as a function of temperature, may be determined for the Planck distribution as:
Txcexmax=hc/5k=2.878xc3x9710xe2x88x923m K
Using this model, and a temperature of 5800 K, the sun""s peak emission occurs at approximately 0.50 microns (i.e., 500 nm), near the middle of the visible region. By comparison, a human body with a surface temperature of about 25xc2x0 C. (xcx9c300 K) has peak emission at approximately 9.6 microns, well into the far infrared region. An object which is hot to the touch at about 100xc2x0 C. (xcx9c375 K) has peak emission at approximately 7.7 microns.
Solar radiation is attenuated, both non-selectively and selectively, by the Earth""s atmosphere by scattering and absorption processes. The atmospheric constituents primarily responsible for scattering are gas molecules (via Rayleigh scattering), particulates, and water droplets. Absorbing molecules in the atmosphere, such as ozone (i.e., O3), water (i.e., H2O), oxygen (i.e., O2), and carbon dioxide (i.e., CO2) absorb substantial solar intensity at specific wavelengths. The amount of atmospheric absorption depends on the length of the radiation path through the atmosphere, which can be characterized by the so-called xe2x80x9cair mass,xe2x80x9d denoted m, which is defined as the ratio of path length and thickness of atmosphere, that is, m=1/Cos xcex7z where xcex7z is the angle between the sun""s rays and the normal to the earth""s surface. With the sun directly overhead (e.g., xcex7zxcx9c0xc2x0), the air mass is 1; with the sun at the horizon (e.g., xcex7zxcx9c85xc2x0), the air mass is xcx9c11. At approximately xcex8zxcx9c83xc2x0, only about 10% of visible solar radiation is transmitted.
A graph of the solar intensity outside the atmosphere and at an air mass of one, along with the intensity of emission from a blackbody at 5900 K, all as a function of wavelength from about 200 to about 3200 nm, is shown in FIG. 1 (from Meyers et al., 1987). At sea level, approximately 3% of the total solar irradiance falls in the ultraviolet region (150 to 380 nm), approximately 47% of the total solar irradiance falls in the visible region (380 to 750 nm), approximately 44% falls in the near infrared region (750 to 2000 nm), and approximately 6% falls in the far infrared region (2000 to 10,000 nm).
A common approach to thermal control coatings has been to prepare a coating which transmits most or all of the incident visible radiation, while blocking (reflecting or absorbing) most or all of the incident infrared radiation.
Kirchoff""s Law states that, for a given wavelength, the sum of transmitted intensity (T), reflected intensity (R), and absorbed intensity (A) must equal one; that is, T+R+A=1. Transmitted radiation passes through the material without a substantial change in wavelength or intensity, while reflected radiation is reflected without a substantial change in wavelength or intensity. Absorbed radiation is absorbed by the material, usually resulting in an increase in temperature, and some or all of the absorbed intensity may be re-emitted, typically at lower wavelengths in accordance with the material""s temperature. Thus, for thermal control films, it is often desirable to have high transmission, low reflectivity, and low absorptivity in the visible region, and to have low transmission, high reflectivity, and low absorptivity in the infrared region.
Suitable materials for use in optical coatings, particularly for those which are useful in glazing applications, are preferably non-toxic, inexpensive, easily available, and required in as thin as possible a layer consistent with durability. A variety of materials have been used as optical coatings, and most of these materials can be broadly classed in two categories, those used for their intrinsic electronic properties (e.g., metals), and those used as optical interference films (e.g., dielectrics), and combinations thereof (e.g., metal-dielectrics).
Metals, and metal-like materials, are characterized by intrinsic electronic and electrical properties which may permit the reflection and/or absorption of incident radiation at the wavelength of interest, while permitting the unimpeded transmission of other wavelengths. Metals which have the lowest absorptivity are preferred, and these include the high performance free-electron metals aluminum, silver, and gold, and alloys of these. Other metals are also used, including, for example, rhodium and alloys thereof (see, for example, Austin, 1994). The reflectance of metal layers may be calculated from measured optical constants (i.e., optical admittance constants n and k). For wavelengths throughout the visible and infrared, such metal layers, even very thin metals layers, are typically highly reflective. In addition, such metals films are also highly absorptive. Nonetheless, metal films can be semitransparent for visible radiation and opaque (reflective and/or absorptive) for thermal infrared radiation if they are sufficiently thin. However, due to the film-forming properties of these metals and the need for continuous uninterrupted layers, thicknesses of at least 10 nm are often necessary, thereby limiting the transmission of visible light to less than about 50%. Typically, the metal layer has a thickness of from about 5 nm to about 20 nm. Even with such thin layers, these metal coatings are invariably characterized by a distinct, and often undesirable, metal tint which is visible to the naked eye.
Examples of metal coatings are described in Wildorf, 1976; Murphy, 1979; Dahlen et al., 1980; Hopper, 1981; Granqvist, 1983; Oliver et al., 1987.
The optical and interference properties of thin coatings of dielectric materials have also been exploited for thermal control. Such interference coatings rely on the interaction of incident radiation with one or more boundaries between materials of high and low indices of refraction. The interference effects are caused primarily by partial reflections at boundaries between layers and recombination of the resulting beams in which their phase differences are significant. By exploiting the optical properties of the one or more thin layers with different indices of refraction, it is possible to substantially reduce or substantially increase reflection or transmission of one or more wavelengths, or wavelength bands, from a surface.
Examples of dielectric coatings are described in Hoffman, 1922; Dimmick, 1946; Widdop et al., 1953; Schroder, 1954; Ploke, 1966; Edwards, 1968; Zycha, 1972; Rancourt et al., 1980; Granqvist, 1983; Sato et al., 1984; Ishida, 1988; Kageyama, 1989; Perilloux et al., 1990; Hagindaet al., 1991; Ando etal., 1991; Thelen, 1996.
Coatings which employ a metal film in combination with a thin dielectric film have also been examined, usually to enhance the reflectance of one wavelength band while reducing the reflectance of another wavelength band (e.g., to improve bandpass properties). Thin layers of dielectric materials may be used to xe2x80x9cinducexe2x80x9d transmission through the metal layer (and thereby reduce reflection from the metal layer). Nonetheless, such combination films still suffer from a relatively low transmittance of visible light caused by the metal layers.
Examples of metal-dielectric combination coatings are described in Fan et al., 1982, 1988; Hayashi et al., 1983; Fujimori et al., 1983; Granqvist, 1983; Yatabe etal., 1986, 1987; Phillips etal., 1986, 1995; Meyer etal., 1989; Nistering, 1991; Hood etal., 1991; Woodard, 1993; Hood et al., 1994; Muromachi et al., 1994; Austin, 1994; Belkind et al., 1994, 1996; Wolfe et al., 1996; Mills et al., 1996; Guiselen, 1997.
Until now, the coating materials of choice have not been those which rely on interference effects (e.g., dielectrics), but instead have been those with intrinsic material effects which exhibit optical properties determined by the free electrical conducting carriers within it (e.g., metals), as discussed above. Although multilayer dielectric interference coatings have been examined in the past, they have been largely dismissed as candidates for many practical glazing applications. The need for a relatively thick coating of these brittle materials, coupled with the relatively intensive and expensive methods used for their deposition, has instead led those of skill in the art to develop new materials (e.g., alloys, semiconductors) with improved intrinsic material effects similar to the high performance metals discussed above, rather than pursue dielectric coatings.
The need forflexible thermal control films is well established. Flexible films are useful in a number of applications such as non-planar windows and retrofitting, as well as in the streamlined manufacture of standard windows. A number of such films, which employ primarily metal coatings on polymer sheets and combination metal-dielectric coatings, are commercially available.
Nonetheless, those of skill in the art have generally dismissed the possibility using known relatively thick, brittle, multilayer dielectric interference coatings on flexible substrates. Those of skill in the art have turned away from using dielectric coatings on flexible substrates for a number of practical reasons, including (1) the significant differential in the thermal expansion of the dielectric coating materials and the substrate, which often results in mechanical failure of the film; (2) the intrinsic stress (e.g., compressive, tensile) of the coating materials which can deform the substrate and often leads to a loss of adhesion, and (3) the need to heat the substrate for conventional methods for dielectric deposition, which is detrimental to temperature-sensitive substrates, such as polymeric sheets.
The present invention addresses these and other needs, and pertains to coated polymer sheets which comprise a thin flexible polymeric sheet (which serves as the substrate), and at least one multilayer coating coated thereon, said coating comprising at least two contiguous alternating layers of high and low index of refraction inorganic dielectric material. The coated polymer sheets are characterized by a high transmission of visible radiation (i.e., visibly transparent) and a high reflectance at one or more near infrared radiation center wavelengths (i.e., heat reflective), and rely primarily on the interference effects of the dielectric layers to achieve these results.
One aspect of the present invention pertains to a thermal control film comprising: (a) a thin flexible polymeric sheet having a first face and an opposite second face and a thickness of from 0.1 to 100 mils; and, (b) a multilayer coating adhered to said first face, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material of optical thickness approximately equal to one quarter of an infrared center wavelength, xcex0, between 750 and 2000 nm; said thermal control film characterized by an average transmittance of visible radiation of wavelength 380 to 750 nm at 0xc2x0 incidence of at least 50%; said thermal control film further characterized an average reflectance of infrared radiation in an infrared radiation band which is at least 100 nm wide and falls within the wavelength band 750 to 2000 nm, at 0xc2x0 incidence, of at least 50%.
Another aspect of the present invention pertains to a thermal control film comprising: (a) a thin flexible polymeric sheet having a first face and an opposite second face and a thickness of from 0.1 to 100 mils; and, (b) a first multilayer coating adhered to said first face, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material of optical thickness approximately equal to one quarter of a first infrared center wavelength, xcex1, between 750 and 2000 nm; (c) a second multilayer coating adhered to said second face, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material of optical thickness approximately equal to one quarter of a second infrared center wavelength, xcex2, between 750 and 2000 nm; said thermal control film characterized by an average transmittance of visible radiation of wavelength 380 to 750 nm at 0xc2x0 incidence of at least 50%; said thermal control film further characterized an average reflectance of infrared radiation in an infrared radiation band which is at least 100 nm wide and falls within the wavelength band 750 to 2000 nm, at 0xc2x0 incidence, of at least 50%.
Another aspect of the present invention pertains to a composite thermal control film comprising: (i) a first thermal control film comprising: (a) a first thin flexible polymeric sheet having a first face and an opposite second face and a thickness of from 0.1 to 100 mils; and, (b) a first multilayer coating adhered to said first face of said first sheet, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material of optical thickness approximately equal to one quarter of a first infrared center wavelength, xcex1, between 750 and 2000 nm; (ii) a second thermal control film comprising: (a) a second thin flexible polymeric sheet having a first face and an opposite second face and a thickness of from 0.1 to 100 mils; and, (b) a second multilayer coating adhered to said first face of said second sheet, said coating comprising two or more contiguous alternating layers of high and low index of refraction inorganic dielectric material of optical thickness approximately equal to one quarter of a second infrared center wavelength, xcex2, between 750 and 2000 nm; wherein said first thermal control film is adhered to said second thermal control film; said composite thermal control film characterized by an average transmittance of visible radiation of wavelength 380 to 750 nm at 0xc2x0 incidence of at least 50%; said thermal control film further characterized an average reflectance of infrared radiation in an infrared radiation band which is at least 100 nm wide and falls within the wavelength band 750 to 2000 nm, at 0xc2x0 incidence, of at least 50%.
Another aspect of the present invention pertains to a glazing assembly comprising: (i) a glazing substrate having at least one face; and, (ii) a thermal control film or a composite thermal control film, as described herein, adhered to said face. Another aspect of the present invention pertains to a glazing assembly comprising: (i) a first glazing substrate; (ii) a second glazing substrate; and, (iii) a thermal control film or a composite thermal control film, as described herein, positioned between said first and second glazing substrates, and adhered to said first and second glazing substrates. Another aspect of the present invention pertains to a glazing assembly comprising: (i) a first glazing substrate; (ii) a second glazing substrate; and, (iii) a thermal control film or a composite thermal control film, as described herein, positioned between said first and second glazing substrates, but separated from said first and second glazing substrates by layer of a gas.
Additional preferred embodiments are described below. As will be appreciated by one of skill in the art, features of one aspect or embodiment of the invention are also applicable to other aspects or embodiments of the invention.